2.4 OPERATION OF CELLULAR SYSTEMS

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1 INTRODUCTION TO CELLULAR SYSTEMS 41 a no-traffic spot in a city. In this case, no automotive ignition noise is involved, and no cochannel operation is in the proximity of the idle-channel receiver. We found that in some areas the noise level is 2 to 3 db higher than 120 dbm at the cell sites and 3 to 4 db higher than 120 dbm at the mobile stations Emission Noise Above the 800 MHz. Up to the operating frequency of 3 GHz, the emission noise level may remain the same level as that at the 800 MHz. Usually, the emission noise can be ignored because the interference level caused by the cochannels and adjacent channels is much higher than the emission noise level Amplifier Noise A mobile radio signal received by a receiving antenna, either at the cell site or at the mobile unit, will be amplified by an amplifier. We would like to understand how the signal is affected by the amplifier noise. Assume that the amplifier has an available power gain g and the available noise power at the output is N o. The input signal-to-noise (S/N) ratio is P s /N i, the output signal-to-noise ratio is P o /N o, and the internal amplifier noise is N α. Then the output P o /N o becomes P o N o gp s g(n i ) + N α P s N i + (N α /g) (2.3-20) The noise figure F is defined as F maximum possible S/N ratio actual S/N ratio at output (2.3-21) where the maximum possible S/N ratio is measured when the load is an open circuit. Equation (2.3-21) can be used for obtaining the noise figure of the amplifier. F P s/ktb N o P o /N o (P o /P s )ktb N o g(ktb) (2.3-22) Also substituting Eq. (2.3-20) into Eq. (2.3-22) yields F P s /ktb P s /[N i + (N α /g)] N i + (N α /g) ktb (2.3-23) The term ktb is the thermal noise as described in Sec The noise figure is a reference measurement between a minimum noise level due to thermal noise and the noise level generated by both the external and internal noise of an amplifier. 2.4 OPERATION OF CELLULAR SYSTEMS Operation Procedures This section briefly describes the operation of the cellular mobile system from a customer s perception without touching on the design parameters. 13,14 The operation can be divided into four parts and a handoff procedure.

2 42 CHAPTER TWO Mobile unit initialization. When a user activates the receiver of the mobile unit, the receiver scans the set-up channels. It then selects the strongest and locks on for a certain time. Because each site is assigned a different set-up channel, locking onto the strongest set-up channel usually means selecting the nearest cell site. This self-location scheme is used in the idle stage and is user-independent. It has a great advantage because it eliminates the load on the transmission at the cell site for locating the mobile unit. The disadvantage of the self-location scheme is that no location information of idle mobile units appears at each cell site. Therefore, when the call initiates from the land line to a mobile unit, the paging process is longer. For a large percentage of calls originates at the mobile unit, the use of self-location schemes is justified. After a given period, the self-location procedure is repeated. When land-line originated calls occur, a feature called registration is used. Mobile originated call. The user places the called number into an originating register in the mobile unit, and pushes the send button. A request for service is sent on a selected set-up channel obtained from a self-location scheme. The cell site receives it, and in directional cell sites (or sectors), selects the best directive antenna for the voice channel to use. At the same time, the cell site sends a request to the mobile telephone switching office (MTSO) via a high-speed data link. The MTSO selects an appropriate voice channel for the call, and the cell site acts on it through the best directive antenna to link the mobile unit. The MTSO also connects the wire-line party through the telephone company zone office. Network originated call. A land-line party dials a mobile unit number. The telephone company zone office recognizes that the number is mobile and forwards the call to the MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit number and the search algorithm. Each cell site transmits the page on its own set-up channel. If the mobile unit is registered, the registered site pages the mobile. The mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site. The mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert. Call termination. When the mobile user turns off the transmitter, a particular signal (signaling tone) transmits to the cell site, and both sides free the voice channel. The mobile unit resumes monitoring pages through the strongest set-up channel. Handoff procedure. During the call, two parties are on a traffic channel. When the mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak. The current cell site requests a handoff. The system switches the call to a new frequency channel in a new cell site without either interrupting the call or alerting the user. The call continues as long as the user is talking. The user does not notice the handoff occurrences. Handoff was first used by the AMPS system, then renamed handover by the European systems because of the different meanings in British English and American English. Description of handoff will appear in Chap Maximum Number of Calls Per Hour Per Cell To calculate the predicted number of calls per hour per cell Q in each cell, we have to know the size of the cell and the traffic conditions in the cell. The calls per hour per cell is based on how small the theoretical cell size can be. The control of the coverage of small cells is based on technological development.

3 INTRODUCTION TO CELLULAR SYSTEMS 43 FIGURE 2.12 To establish the traffic capacity from a geographic map (west Los Angeles). We assume that the cell can be reduced to a 2-km cell, which means a cell of 2-km radius. A 2-km cell in some areas may cover many highways, and in other areas a 2-km cell may only cover a few highways. Let a busy traffic area of 12 km radius fit seven 2-km cells. The heaviest traffic cell may cover 4 freeways and 10 heavy traffic streets, as shown in Fig A total length of 64 km of 2 eight-lane freeways, 48 km of 2 six-lane freeways, and 588 km of 43 four-lane roads, including the 10 major roads, are obtained from Fig Assume that the average spacing between cars is 10 m during busy periods. We can determine that the total number of cars is about 70,000. If one-half the cars have car phones, and among them eight-tenths will make a call (η c 0.8) during the busy hour, there are 28,000 calls per hour, based on an average of one call per car if that car phone is used. The maximum predicted number of calls per hour per a 2-km cell Q is derived from the above scenario. It may be an unrealistic case. However, it demonstrates how we can calculate Q for different scenarios and apply this method to finding the different Q in different geographic areas.

4 44 CHAPTER TWO Maximum Number of Frequency Channels Per Cell The maximum number of frequency channels per cell N is closely related to an average calling time in the system. The standard user s calling habits may change as a result of the charging rate of the system and the general income profile of the users. If an average calling time T is 1.76 min and the maximum calls per hour per cell Q i is obtained from Sec , then the offered load can be derived as A Q i T erlangs (2.4-1) Assume that the blocking probability is given (see Appendix A), then we can easily find the required number of radios in each cell. 15 EXAMPLE 2.1 Let the maximum calls per hour Q i in one cell be 3000 and an average calling time T be 1.76 min. The blocking probability B is 2 percent. Then we may use Q from Eq. (2.4-1) to find the offered load A A 88 With the blocking probability B 2 percent, the maximum number of channels can be found from Appendix A as N 100. EXAMPLE 2.2 If we let Q i 28,000 calls per cell per hour, based on one scenaro shown in Sec , B 2 percent, and T 1.76 min, how many radio channels are needed? The offered load A is obtained as 28, A 821 Inserting the above known figures into the table of Appendix A, we find that N 820 channels per cell. EXAMPLE 2.3 If there are 50 channels in a cell to handle all the calls and the average is 100 s per call, how many calls can be handled in this cell with a blocking probability of 2 percent? Because N 50 and B 2 percent, the offered load can be found from Appendix Aas The number of calls per hour in a cell is Q i A calls per hour EXAMPLE 2.4 If the maximum number of calls per hour per cell is 1451 and there is a seven-cell reuse pattern * in the system (K 7), and assuming that B 2 percent and T 100 s as in Example 2.3, then N 50 as indicated. The total number of required channels for a K 7 reuse system is N t radios * Its pattern is shown in Fig and described in Sec

5 INTRODUCTION TO CELLULAR SYSTEMS 45 If a large area is covered by 28 cells, K t 28; the total number of customers M t K t i1 M i in the system increases. Therefore, we may assume that the number of subscribers per cell M i is somehow related to the percentage of car phones used in the busy hours (η c ) and the number of calls per hour per cell Q i as M i f (Q i,η c ) (2.4-2) where the value Q i is a function of the blocking probability B, the average calling time T, and the number of channels N. Q i f (B, T, N) (2.4-3) If the K 7 frequency reuse pattern is used, the total number of required channels in the system is N t 7 N. We must realize that it is the maximum number of calls per cell Q i that determines the total required channels N t, not the total number of subscribers M t.in this case (K t 28 and K 7), the total number of channels N t has been used four times in the system. 2.5 CONCEPT OF FREQUENCY REUSE CHANNELS A radio channel consists of a pair of frequencies, one for each direction of transmission that is used for full-duplex operation. A particular radio channel, say F 1, used in one geographic zone as named it a cell, say C 1, with a coverage radius R can be used in another cell with the same coverage radius at a distance D away. Frequency reuse is the core concept of the cellular mobile radio system. In this frequency reuse system, users in different geographic locations (different cells) may simultaneously use the same frequency channel (see Fig. 2.13). The frequency reuse system can drastically increase the spectrum efficiency, but if the system is not properly designed, serious interference may occur. Interference due to the common use of the same channel is called cochannel interference and is our major concern in the concept of frequency reuse Frequency Reuse Schemes The frequency reuse concept can be used in the time domain and the space domain. Frequency reuse in the time domain results in the occupation of the same frequency in different time slots. It is called time-division multiplexing (TDM). Frequency reuse in the space domain can be divided into two categories. FIGURE 2.13 The ratio of D/R.